Centrifugal separators, for example those used in separating blood components, can employ a disposable plastic channel that is fitted within a centrifuge bowl driven by a motor. These channels typically have a beginning with an inlet for whole blood and an end where most of the separated components are removed by separate outlets, the beginning and the end being located next to each other but isolated from each by a plastic wall preventing mixing of the incoming liquid with that at the end of the channel.
For example, Kellogg et al. U.S. Pat. No. 4,094,461 discloses a single-stage, blood separation channel of generally constant radius in which a whole blood inlet is provided at the beginning and all of the separated components are removed from a collection chamber at the end of the channel, the beginning and end being separated by a wall. In the collection chamber, a dam is placed behind a white cell/platelet outlet to block flow past it of the white cells and platelets of interest but to permit flow of the heavier red cells and lighter plasma. On the other side of the dam, an interface positioning outlet is provided for the purpose of maintaining the position of the interface between the red cells and plasma in order to control the position of the thin white cell/platelet layer at the white cell/platelet outlet to provide efficient white cell/platelet removal.
In my U.S. Pat. No. 4,386,730, there is shown a two-stage separation channel having a constant-radius first-stage separation portion wherein the separated red blood cells flow along the outer wall back toward an outlet near the beginning of the channel, and the platelets and plasma continue beyond the first-stage portion, through a transition portion with a decreasing-radius outer wall, and into a radially-increasing second-stage separation portion with a plasma outlet and a platelet outlet at its end. Once again the beginning and the end of the channel are separated from each other by a wall. In operation, it is necessary that the interface between the red blood cells and the separated plasma and platelets be maintained at the transition portion by continuous monitoring and adjusting of flowrates by an operator.